static electricity and charge...

Safety Engineering

-Static Electricity and Charge Accumulation-

Conductive– A material with a low electrical resistance, electrons flow

easily across the surface or through the bulk of thesematerials and having a volume resistively equal or lowerthan 104Ω∙m

Dissipative– A material incapable or retaining a significant amount of

electrostatic charge when in contact with earth and having avolume resistivity higher than 104Ω∙m but equal to or lowerthan 109Ω∙m measured at ambient temperature and 50%relative humidity.

Non-conductive– A material prevent or limit the flow of electrons across its

surface or through its volume. Non-conductive materialshave a high electrical resistance and are difficult to groundand having a volume resistivity higher than 109Ω ∙ m

Definitions - Types of materials 2

• Generation of Spark Discharges.

– Charge accumulation at a conductive object.

– Field strength exceeds the electric strength of the ambient atmosphere.

• Ignitability--gases, vapors, dusts

• Energy transfer--up to 10,000 mJ

Types of discharges 3

• Discharging of static electricity between two conductors.

Spark Discharge

• Generation of Brush Discharges

– Conductive electrode moves towards a charged nonconductive object.

• Nonconductive lining or surface must have a breakdown voltage greater than 4 kV and a thickness greater than 2 mm.

• Nonconductive coating can be a layer of the powdered solid.

• Ignitability--gases, vapors

• Energy transfer--up to 4 mJ

4

Brush Discharge

Types of discharges

5

• Generation of Propagating Brush Discharge

– Bipolar charging of the high resistivity material (non conducting) that is lining another conductor.

– Field strength exceeds the electric strength of the high resistivity material. Non conducting lining must have breakdown voltage greater than 4 kV

• Ignitability--gases, vapors, dusts

• Energy transfer--up to 100,000 mJ

• Major contributor to static electricity ignitions.

Propagating Brush Discharge

Types of discharges

• Generation of Cone Discharge.– Vessels larger than 1 m3.– Relatively fast filling rate, greater

than 0.5 kg/s.– High resistivity (>1010Ωm) bulk

product, larger than 1 mm diameter.

– Charge accumulation in the bulk product.

– Field strength exceeds the electric strength of the ambient atmosphere.

• Ignitability--gases, vapors, dusts• Energy transfer--up to 1000 mJ

6

Cone Discharge

Types of discharges

Type of Discharge Energy transfer Ignitability

Spark < 10 000 mJ gases, vapor, dusts

Brush < 4 mJ gases, vapor

Propagating Brush < 100 000 mJ gas, vapor, dusts

Cone < 1 000 mJ gas, vapor, dusts

Corona < 0.1 mJ some gases with

low MIE

Ignitability of discharges 7

Whenever two dissimilar materials come in contact, electrons move from one surface to the other. As these materials are separated and more electrons remain on one surface than the other, one material takes on a positive charge and the other a negative charge.

Mechanisms for Charge Accumulation:

– Contact and Frictional

– Double layer

– Induction

– Transport

Charge Accumulation 8

• Dust transport– e.g. pneumatic transport of powders/solids

• Pouring powders– e.g. pouring solids down chutes or troughs

• Gears and belts– e.g. transporting charges from one surface to another


Contact and Frictional Charging

Double layer charging

Caused by friction and movement at interfaces on a microscopic scale.

– Liquid-liquid

– Solid-liquid

– Solid-solid

– Gas-liquid

– Gas-solid

When an isolated conductor is subject to a electric field a chargepolarity develops on the object. If the object is grounded then thecharges closest to the grounding source flows away leaving the bodywith a net charge of opposite sign.


Induction charging

Charging by Transport

• Results from a charged dust, liquid or solid particles settling onto a surface and transporting their charges to this new surface.

• The rate of charge accumulation is a function of the rate of transportation.

Many fluid handling operationscan generate static electricity.This becomes a problem whennon conducting pipes (glass orTeflon lined) are used withoutadequate bonding.

Charging by Transport 11

Fluid handling operations

Splash Filling

• When non conducting fluids (or solids) free fall through air they pick up a significant static charge.

• When there is spraying or splashing static electricity can build up.

• This can be a source of sparks

• When fluid flows into a vessel it carriesa charge with it which can build up inthe tank if the tank is not properlygrounded.

• Routine inspection of groundingminimizes the change for fire orexplosion due to a spark discharge fromthe charged tank.


Fluid flow into vessels

When a liquid or solid isflowing, there is a transfer ofelectrons from one surface toanother as they flow past eachother.


Streaming current

The time required for a chargeto dissipate by leakage.


Relaxation time

1 mho/centimeter [mho/cm] = 100 siemens/meter [S/m]

Example:toluene- poor conductivity- relaxation time: 21 s

For flow through a non conducting pipe(glass, Teflon lined) a voltage drop candevelop from flowing liquid.


Electrostatic Voltage Drops

Charges can accumulate as a result of streaming current

Assuming constant streaming current

Solid geometries are almost always ill defined, so need to be based onempirical calculations. Solid processing operations have differentempirically determined charge capacities.

Q = Charge Capacity X Charge Rate X time coulombs kg

Q skg s


Accumulated charge from solid handling

Process Charge Capacity (coulombs/kg)

Sieving 10-9 to 10-11

Pouring 10-7 to 10-9

Grinding 10-6 to 10-7

Sliding down incline 10-5 to 10-7

Pneumatic transport 10-5 to 10-7

Capacitance 17

Capacitance is the ability of a body to store an electrical charge.Capacitance for a sphere for plates

Object Cpacitance (farad)Small scoop 5 x 10-12

Bucket 10 x 10-12

Barrel 100 x 10-12

Person 200 x 10-12

Automobile 500 x 10-12

Tank Truck 1000 x 10-12

Capacitance of Various Objects

Static Energy Stored

• Determine the capacitance, C, of the object or container contents, expressed in farads or coulombs per volt.

• Determine the accumulated charge, Q, expressed in coulombs

• Compute accumulated energy, E, expressed in J or mJ.

• Compare to the MIE of the dust or vapor.

Calculations 18

Determine the potential hazard of pneumatically transporting a dry powder (dry powder with a particle size greater than 1 mm) at a rate of 30,000 kg/hr into a metal vessel which has a volume of 70 m3.

Given: The powder has a bulk density of 600 kg/m3; the vessel has a spherical geometry; 70 m3 of powder is charged into the vessel. The powder is flammable with a MIE of 20 mJ.

Example – Solids handling

• Determine radius of sphere:

• Calculate capacitance:

113 333 3 70

2.54 4

V mr m

0

12 10

4

= 1 for air (Table 7-1) Spark jumps across air gap

4 (1) 8.85 10 2.5 2.83 10

r

r

C r

coul coulC mvolt m volt

Example – solids handling - solution 19

(Tab. slide15)

• Determine mass fed:

• Calculate charge accumulated (Tab. slide 17, pneumatictransport):

3

370 600 42,000

kgFeed m kg

m

510 42,000 0.42coulQ kg coulkg

Example – solids handling – solution (cont.) 20

• Calculate energy:

• This is much greater than the MIE of the powder. If there is sufficient air this would be very hazardous.

• This is the total charge that could go into vessel while filling. Multiple discharges would occur, certainly there would be conical pile discharges (unless grounded).

228

10

0.423.1 10

2 2 2.83 10

coulQE J

coulCvolt

• Charge buildup is always possible when you have moving fluids or solids. The potential for discharge is always present.

• We can eliminate sparks if we ensure that all parts of the system are connected with a conductor.

• Historically there was little problem when piping was all copper, stainless steel or iron. The problem comes when pipes or vessels are glass or Teflon lined or made from polymers or connected with non-conducting gaskets.

• There has always been a problem when you are pouring either liquid or a solid through an open space i.e., a filling operation.

Elimination of Charge Accumulation 21

Bonding and Grounding

• Bonding– Is the connection of a

conducting wire between two or more objects.

– The voltage difference between the two objects is reduced to zero, however they may have a voltage difference relative to ground or another non connected object

• Grounding– Is the connection of a

conducting wire between a charged object and the ground.

– Any charge accumulated in the system is drained off to ground.


Bonding and Grounding


Bonding


Grounding

• Glass and plastic lined vessels are grounded using tantalum inserts or a metal probe.

• This is less effective if fluid has low conductivity.


Grounding Glass-lined Vessels

• To eliminate the static charge that builds up from a fluid free falling through air, a dip leg is used. Note hole to prevent back siphoning.

• An angle iron can also be used so fluid runs down the angle iron instead of free falling.


Dip Legs to Reduce Splash Filling

Following are a series of case studies of accidents that actually happen at BASF and Dow and shared with the SACHE Chemical Process Safety Workshop participants.

Case Studies from a production plant 30

• Situation– A non-conductive bulk product is

fed out of 25 kg PE-bags in a vessel, in which a flammable liquid is being stirred. During shaking of the the just empty bag an ignition occurred.

• Cause– All handling of non-conductive

solids or bulk products may generate static electricity. Due to contact charging of the sliding bulk product, both the bulk product and non conducting package materials became charged. Brush discharges form the surface of the bad ignited the vapor/air mixture.

• Precaution– Either fill into a closed, inerted

vessel or avoid charge generation.


Solids Filling Operation

• Situation– An operator filled a non-conductive

bulk product out of 25 kg PE-bags in a solvent free mixer. Exhaust system operated. All equipment grounded, the floor was dissipative, the operator wore dissipative footwear. During pouring the product in the reaction vessel explode.

• Cause– The plastic wrap that held the sacks

on the pallet was on the floor and the operator was standing on it. This allowed a static charge to build up in him.

• Precaution– Always guarantee ground

connection.


Operator

• Situation– A ball-valve is installed in a waste

gas collecting system. During usual production an explosion occurred; the pipe system was destroyed.

• Cause– A valve consists of conductive and

non-conductive parts. Conveying of dust suspensions or droplets may generate charge accumulation on the ball and/or shaft if not bonded to the grounded housing. Spark discharge from charged ball to housing caused explosion.

• Precaution– Guarantee ground connection of

conductive equipment.


Valve

• Situation– A pure liquid was filled in a steel drum

with an inner plastic liner. To avoid splash filling a short funnel was inserted in the spout. The nozzle, the drum and the weighing machine were all grounded. Despite having an exhaust system there was an explosion during drum filling.

• Cause– Electrostatic charge generation at the

surface of the non-conductive coating cannot be transferred. The funnel had sufficient capacitance was insulated from the ground by the PE lined filler cap. Spark discharge from funnel caused explosion.

• Precautions– Guarantee ground connection of all

conductive equipment.


Lined metal drum filling

Example – Fluid Handling

• Determine the voltage developed between a charging nozzle and a grounded tank and the charge accumulated during the filling process at 150 gpm.

Example – Fluid Handling (cont.)

• Additional information:

– Non conducting hose length 20 ft

– Hose diameter 2 in.

– Liquid conductivity 10-8 mho/cm

– Liquid diffusivity 2.2x10-5 cm2sec-1

– Dielectric constant 25.7

– Density 0.88 g/cm3

– Viscosity 0.60 centipoise

– MIE 0.10 mJ

Example – fluid handling - solution

• Procedure

• Calculate voltage drop using V=IsR (Eq. 7-17)

• Calculate R using Eq. 7-18

• Calculate Is using Eq. 7-12 or 7-14

• Calculate Q using Q=Ist

• Calculate E=(QV/2)

• Compare to MIE

Example – fluid handling – solution (cont.)

Calculate the Resistance

222 2

9

82

12 . 2.54(20 ) 610.

3.541 . 20.3.

6103.00 10

10 20.3C

in cmL ft cmft in

cmA r in cmin

L cmR

A cmcm


• Determine type of flow (laminar or turbulent)

3 2

2 2

3

3

150 144 1minmin 15.37.48 601 .

2 15.3 0.887750

Re 348,0000.60

Hence Turbulent

gallonft in ft

usgal ft sin

ft gin

du s cpcm

ft gcp ins cm


• Calculate the streaming current:

14

50

8

2-5 5 -5 -5

14

14

25.7 8.85 1022.7 10 (Eq. 7-16)

10

= 2.2 10 22.7 10 = 7.07 10 = 2.78 10 . (Eq. 7-15)

5.89 10 (Eq.7-14)

5.89 10

r

C

m

rs

s

scm s

mhocm

cmD s cm ins

amp d uI

ftvolt

s

ampI

7

-5

2 153 25.7 0.1

1.66 102.78 10

ftin volt

samp

ft involts

Example – fluid handling – solution (cont.)• Calculate the voltage drop, accumulated charge and

energy:

7 9

5 5

5

(1.66 10 ) 3.00 10 498 (Eq. 7-17)

Determine fill time

300 60min 120

150min

Determine accumulated charge

1.66 10 120 1.99 10

Determine Energy

1.99 10

2

s

s

V I R amp volts

sgalt s

gal

Q I t amp s coulomb

couloQVE

4984.9

2

This is greater than the MIE, so there is a fire or explosion hazard

mb voltmJ

Static electricity & charge accumulation

• Definitions

• Types of discharges

• Mechanisms of charge accumulation– fluid systems - Streaming current

– Solids handling

• Balance of charges

• Bonding and grounding

• Case studies

Balance of Charges

• When you have a vessel that has multiple inputs and outputs, you can determine the charge accumulation by a charge balance.

• Consider streaming currents in, charges carried away by flows going out, and charge loss due to relaxation.

Charge Balance

, ,1 1

,1

,1

where

is the current coming into the vessel

is the current flowing out of the vessel

is the charge loss due to relaxation

is the relaxation ti

n m

s si in j outi j

n

s i ini

m

s j outj

dQ QI I

dt

I

I

Q

me

Charge Balance

,

The charge flowing out of the vessel depends

on the charge already in the tank

where

is the rate of discharge through outlet

is the container or tank volume

is the total char

j

s j outC

j

C

FI Q

V

F j

V

Q

ge in the tank

Charge Balance

,1 1

,

Hence the charge balance becomes

If flows, , streaming currents, , and relaxation times, ,

are constant, then this is a linear di

n mj

s i ini j C

j s i in

FdQ QI Q

dt V

F I

, ,

1 10

1

1 1

fferential equation that has

the solution:

where

1

1 1

Ct

n n

s s mi in i inji i

m mj Cj j

j jC C

Q A Be

I IF

A B Q CVF F

V V

Charge Balance

• This relationship is used to determine the charge developing in the tank as a function of time relative to an initial charge of Q0.

• The capacitance of the vessel is calculated as before (typically assume equivalent spherical vessel).

• The static energy stored in the vessel is then calculated from E=Q2/2C.

• Examples 7-9 and 7-10 demonstrate using this relationship.

Static electricity & charge accumulation

• Definitions

• Types of discharges

• Mechanisms of charge accumulation– fluid systems - Streaming current

– Solids handling

• Balance of charges

• Bonding and grounding

• Case studies

static electricity and charge...

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